Recently, so-called rear-projection-type projectors have been attracting attention that forms an optical image on a liquid crystal panel (LCD: liquid crystal display), emits light to the optical image and projects the optical image through a projector lens system onto a screen so as to be enlarged. As a method of obtaining color images, it is known to combine images on three liquid crystal panels by use of a dichroic prism while illuminating the three liquid crystal panels with a red, green and blue rays, respectively. For the projector lens system, to reduce cost, plastic is frequently used as the lens material. Since plastic lenses are inferior to glass lenses in environmental capability, it is necessary to set the lens arrangement with the optimum optical power (refractive index) and set the optimum material in consideration of the temperature characteristic of the lens material. On the other hand, to attain a smaller size, a higher resolution and a larger screen in projectors, the pixel pitch of liquid crystal panels has been more and more decreasing, and projector lens systems are required of high optical performance such as small lateral chromatic aberration and low distortion.
Conventionally, as lens systems for projection, various retrofocus optical systems have been proposed that include from the screen side: a first lens unit that includes negative first and second lenses each having an aspherical surface, and whose overall focal length is negative; and a second lens unit having an aspherical surface and whose overall focal length is positive. Examples of such optical systems include ones described in Japanese Laid-Open Patent Application No. 2000-305012 (pages 4 to 18, FIG. 1) and Japanese Laid-Open Patent Application No. 2003-156683.
Patent document 1: Japanese Laid-Open Patent Application No. 2000-305012 (pages 4 to 18, FIG. 1)
Patent document 2: Japanese Laid-Open Patent Application No. 2003-156683 (pages 5 to 14, FIG. 8)